The CMS electromagnetic calorimeter: status, performance with cosmic and first LHC data

The Compact Muon Solenoid (CMS) detector at the Large Hadron Colider (LHC) is ready for first collisions. Electrons and photons clean identification and excellent energy and momentum resolution are crucial at LHC in several fields. They are essential in at least two of the Higgs decay channels, they can be signatures of the decay of new heavy bosons, they play a role in Supersymmetry and are of course central in the reconstruction of electroweak and QCD processes. Therefore a key design feature of the experiment is ECAL, the high resolution electromagnetic calorimeter made of 76,000 lead tungstate crystals. The design goal for ECAL is the potential to discover a neutral Higgs boson in the low mass region by measuring the decay into two photons. For a low mass Higgs the intrinsic decay width is very small, therefore the measured width precition is dominated by the ECAL energy resolution. This has led to a target energy resolution of 0.5% for electrons and photons above 100 GeV. The design and performance of the CMS ECAL with test beams, cosmic rays, and first beam dump events at the LHC in 2008 will be presented. In addition, the status of the calorimeter and plans for calibration with first collisions will be discussed. CMS ECAL is ready for exciting physics with LHC and design goals are within reach.